The present invention, in some embodiments thereof, relates to improving the performance of motor servo-drives that feed servo-motors with current, and more particularly but not exclusively to improving current sensitivity across a whole current range, even if that range is large. The motor produces force (or torque) and acceleration that are directly related to the current. Better current enables more accurate velocity and position control of servo-motors. The more accurate the output current of the drive (that feeds the motor) follows the command; the more accurate is the velocity and position control of the drive. The improved performance of the drive is achieved by improving the signal to noise ratio of the current measurement especially when the required current is of low level relative to the maximum current that is needed when accelerating the motor, as an example.
It is well known that in order for a motor drive to provide the best performance possible, a closed loop current control method is used. The drive utilizes current sensing circuitry that produces an electrical signal that represents the actual output current of the drive. The measured output current is compared to the desired current command and the error between the two is used by the drive to correct the output current.
It is well known that the correction to the output current is limited by the accuracy of the measurement and the extent that the current measured in fact represents the actual output. Any deviation of the measured signal from the actual output current may produce a false correction by the drive. Such deviations are typically due to quantization noise (or errors), electro-magnetic interference (EMI) from the surrounding electrical circuits and other sources of noise.
It is well known that in order to achieve a high performance velocity and position control of the motor, a closed loop method is used. The actual velocity and/or position are measured and compared to the desired velocity and/or position. The deviation between the desired velocity and the actual measured velocity, the velocity error, or for that matter the deviation between the desired and measured position—the position error—is used as a correction to the command to the motor servo-drive (=drive command). Any noise, say quantization noise, in the motor current may introduce noise into the force or torque that the motor produces and may thus negatively affect how well the actual velocity and the actual position follows the desired velocity and position.
There are two main types of motor servo-drives available: linear drives and switching Pulse Width Modulation (PWM) drives. The drives may operate many types of motors: single phase and multi-phase motors, both of linear and rotary types. The motors may be of any type of motor structure, including DC motors, permanent magnet synchronous motors, asynchronous induction motors, voice coils, stepper motors, etc.
There are some differences between linear drives and PWM drives in particular which are now summarized.
It is well known that linear drives suffer from low efficiency relative to a PWM type of drive with the same output power capability, and therefore the linear drive dissipates a significant amount of heat by comparison. The linear drives are relatively bigger and typically more expensive. Linear drives are also quieter than PWM drives, producing less electro-magnetic noise that affects the current sensing circuitry. As a result linear drives are able to feed the motor with current that better replicates the desired current, over a wider range of currents, especially when low level of currents are needed for delicate velocity and position correction.
There are numerous high accuracy positioning applications that require a combination of high dynamics, accelerations (in the order of a few g) of large mass or inertia, and therefore high currents, with low standstill position jitter, that is to say deviations from the desired position at standstill, of a few nanometers and below a nanometer, and a following error of a few nanometers and below while moving at a constant speed, when very low level of currents are required just for correction of such small errors.
Examples of such applications include wafer inspection and metrology systems that utilize high accuracy positioning tables, both of air-bearing type and mechanical bearing types. Such positioning stages may include systems with a single linear motor and position feedback, and also gantry axis systems, which utilize two motors and two position feedbacks per each gantry axis. State of the art wafer inspection and metrology systems require standstill jitter at nanometer and sub-nanometer levels, and following errors while moving at a constant speed of a few nanometers. Such applications, or similar, also use high accuracy rotary positioning tables that require a combination of high accelerations of a relatively high inertia and therefore high currents and low standstill jitter of a few micro degrees and below.
For positioning applications that demand a combination of high dynamics and nanometer and sub-nanometer, or microdegree and sub-microdegree, standstill jitter and low following error, during constant velocity or at standstill, the only viable servo-drive in the current art is the linear drive. More particularly, existing PWM servo-drives are typically not used in applications that require jitter and following errors during standstill, of less than about 10 nanometers. At constant velocity the following error that can be achieved with PWM drives is typically worse.